Hi @karnawal ,
What is the make and model number of the flat iron.
It could be the temperature sensor in the iron or the iron’s temperature control circuit.
The iron would have to be opened and tested to find out what the problem is.
The hard part may be finding suitable replacement parts if it is a faulty component as sometimes the device may be designed to be non-repairable by the manufacturer and no spare parts are available, but you can only test it first to know what the problem is.
You will need to have a DMM (digital multimeter) and know how to use it and also have some knowledge about electronic circuits and components.